This invention relates to a mold-press forming glass and a method of producing the same. Throughout the specification, the term xe2x80x9cmold-press forming glassxe2x80x9d means a glass for use as a material to be press formed into a glass product by the use of a mold.
A mold-press forming method of forming a glass material into a glass product is a method capable of obtaining the glass product having a high surface accuracy without requiring a cutting operation and a polishing operation. The mold-press forming method is often used in production of an aspheric lens, a substrate for a magnetic recording medium, and the like. Production of the glass product by the mold-press forming method is carried out in the following manner. In a mold having a mold shape obtained by precision working, the glass material is heated to a temperature at which the glass material is softened and deformed. Thereafter, the glass material is press formed to produce the glass product with the mold shape transferred thereto.
The mold used in mold-press forming is selected so as to meet the following requirements. That is, the mold can be subjected to precision working at a high surface accuracy and is not changed at a molding temperature. As a material of the mold, cemented carbide such as WC (tungsten carbide) is predominantly used. In a case using WC as the material of the mold, Co, Ni, or Cr is often used as a binder. In view of corrosion resistance, sintering may be carried out by the use of a small amount of TiC or TaC.
When used under a high-temperature condition, the mold may be made of a ceramic material such as SiC. In case where SiC is used as the material of the mold, the mold is subjected to surface polishing after sintering. In order to further improve the surface accuracy, surface voids or recesses of the mold may be buried by CVD.
Depending upon the pressing condition and the type of the molded product, various types of SUS (Steel Use Stainless) may be selected.
In order to improve the quality of the product and the mass producibility in mold-press forming, it is essential that the mold and the glass are not fused or fusion-bonded to each other. Furthermore, it is important that, even after repeated use, the surface accuracy of the mold is kept.
However, depending upon the species of the glass, the mold and the glass may be fused to each other after repeated use or, in an extreme case, after a single cycle of forming. If the glass and the mold are fused, the glass is adhered to the surface of the mold. In this event, production may become impossible or, if production can be continued, the surface accuracy of the mold is degraded so that a product exactly as designed can not be obtained. Therefore, in order to improve the quality of the product and the mass producibility, it is important to use a glass which is not fused to the mold.
In order to prevent the glass from being fused, use is practically made of a method of coating the surface of the mold with a releasing or separating film. For example, the releasing film is made of a material selected from pure metal, such as Cr, Ni, W, Pt, Ir, and Au, an alloy or a carbide thereof, C, TiCN, TiAIN, TiN, and BN. However, it is desired to carry out mold-press forming without coating the mold with the parting film.
It is therefore an object of the present invention to provide a mold-press forming glass which is not fused to a mold during mold-press forming.
It is another object of the present invention to provide a mold-press forming glass which is free from reliance or dependence upon a mold and which does not require a parting film to be formed on the mold, thereby reducing a production cost.
It is still another object of the present invention to provide a mold-press forming glass capable of improving the mass producibility of mold-press forming so that press products can be produced at a low cost.
It is yet another object of the present invention to provide a mold-press forming glass capable of providing a product having a high surface accuracy.
Other objects of the present invention will become clear as the description proceeds.
In order to achieve the above-mentioned objects, the present inventor has made extensive studies and researches to find out that the basicity of the glass essentially dominates occurrence of fusion and hereby proposes the present invention.
According to an aspect of the present invention, there is provided a mold-press forming glass having the basicity adjusted to be equal to 11 or less.
According to another aspect of the present invention, there is provided a method of evaluating a mold-press forming glass. The evaluating method comprises the steps of preparing a candidate composition, calculating the basicity of the candidate composition as a calculated basicity, and evaluating the fusibility with a mold with reference to the calculated basicity.
According to still another aspect of the present invention, there is provided a mold-press forming glass evaluated by the evaluating method.
According to yet another aspect of the present invention, there is provided a method of designing a mold-press forming glass. The designing method comprises the steps of preparing a candidate composition, calculating the basicity of the candidate composition to produce a calculated basicity, evaluating the fusibility with a mold with reference to the calculated basicity to produce an evaluation result, and determining a glass composition for the mold-press forming glass with reference to the evaluation result.
According to a further aspect of the present invention, there is provided a mold-press forming glass designed by the designing method.
According to a still further aspect of the present invention, there is provided a method of producing a mold-press forming glass. The producing method comprises the steps of preparing a candidate composition, calculating the basicity of the candidate composition to produce a calculated basicity, evaluating the fusibility with a mold with reference to the calculated basicity to produce an evaluation result, determining a glass composition for the mold-press forming glass with reference to the evaluation result, preparing a material having the glass composition, melting the material into a molten material, and forming the molten material into a predetermined shape.
According to a yet further aspect of the present invention, there is provided a mold-press forming glass produced by the producing method.
In this invention, the basicity is defined by (Total Mole Number of Oxygen Atoms)/(Total Field Strength of Cations)xc3x97100. xe2x80x9cField Strengthxe2x80x9d (hereinafter abbreviated to F.S.) in the formula is given by:
F.S.=Z/r2,
where Z represents the ionic valence, r, the ionic radius. For the values of Z and r in this invention, reference is made to xe2x80x9cChemical Handbook, Basic, 2nd Edition (1975. published by Maruzen)xe2x80x9d.
Calculation of the basicity of the glass will be described in conjunction with SiO2 by way of example.
At first, the mole number of oxygen atoms is calculated. 1 mol of SiO2 contains 2 mol of oxygen atoms. By multiplying 2 mol as the mole number of oxygen atoms by mol % of SiO2 in glass composition, the mole number of oxygen atoms of SiO2 in the glass is calculated. Likewise, calculation of the mole number of oxygen atoms is calculated for each component. The total sum of the mole numbers is represented by xe2x80x9cTotal Mole Number of Oxygen Atomsxe2x80x9d.
Next, F.S. is calculated. For a cation Si4+, Z=4 and r=0.4. As a result, F.S.=25. Since 1 mol of Si4+is contained in 1 mol of SiO2, F.S. in the glass is obtained by 25xc3x971 (mol)xc3x97(mol % of SiO2 in the composition).
Similarly, F.S. is calculated for each component. The total sum is represented by xe2x80x9cTotal F.S. of Cationsxe2x80x9d. Thus, xe2x80x9cTotal Mole Number of Oxygen Atomsxe2x80x9d divided by xe2x80x9cTotal F.S. of Cationsxe2x80x9d multiplied by 100 gives xe2x80x9cBasicity of Glassxe2x80x9d.
Description will be made of a mechanism that the basicity of the glass dominates occurrence of fusion.
The basicity of the glass serves as an index representing the degree of attraction of oxygen electrons in the glass by cations in the glass. In the glass high in basicity, attraction of oxygen electrons by the cations in the glass is weak. When the glass high in basicity is contacted with cations which have strong motivation to require electrons and are included in the mold, the cations easily enter from the mold into the glass, as compared with a glass low in basicity. In case where the cations as the mold component enter or diffuse into the glass, the concentration of the mold component in a glass phase near an interface or boundary between the mold and the glass is increased. In this event, the difference in composition between the glass phase and the mold phase is reduced so that the affinity between the glass and the mold is increased. Accordingly, the glass is easily wet with the mold. It is believed that the glass is fused to the mold by the above-mentioned mechanism. Thus, as the basicity is lower, the mold component less easily enters into the glass so that the occurrence of fusion between the glass and the mold component is avoided.
In case where WC is used as the mold, the fusion is not caused to occur if the basicity of the glass is not higher than 11, preferably, not higher than 9.5. If the basicity of the glass exceeds 9.5, the glass tends to be fused with the mold. If the basicity of the glass exceeds 11, the glass is fused with the mold. In this event, the surface accuracy of the product is degraded and the mass producibility is significantly decreased.